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Hauck, Judith; Völker, Christoph; Wolf-Gladrow, Dieter A; Laufkötter, Charlotte; Vogt, Meike; Aumont, Olivier; Bopp, Laurent; Buitenhuis, Erik Theodoor; Doney, Scott C; Dunne, John; Gruber, Nicolas; Hashioka, Taketo; John, Jasmin; Le Quéré, Corinne; Lima, Ivan D; Nakano, Hideyuki; Séférian, Roland; Totterdell, Ian J (2015): On the Southern Ocean CO2 uptake and the role of the biological carbon pump in the 21st century, links to supplementary material. PANGAEA, https://doi.org/10.1594/PANGAEA.849079, Supplement to: Hauck, J et al. (2015): On the Southern Ocean CO2 uptake and the role of the biological carbon pump in the 21st century. Global Biogeochemical Cycles, 29(9), 1451-1470, https://doi.org/10.1002/2015GB005140

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Abstract:
We use a suite of eight ocean biogeochemical/ecological general circulation models from the MAREMIP and CMIP5 archives to explore the relative roles of changes in winds (positive trend of Southern Annular Mode, SAM) and in warming- and freshening-driven trends of upper ocean stratification in altering export production and CO2 uptake in the Southern Ocean at the end of the 21st century. The investigated models simulate a broad range of responses to climate change, with no agreement ona dominance of either the SAM or the warming signal south of 44° S. In the southernmost zone, i.e., south of 58° S, they concur on an increase of biological export production, while between 44 and 58° S the models lack consensus on the sign of change in export. Yet, in both regions, the models show an enhanced CO2 uptake during spring and summer. This is due to a larger CO 2 (aq) drawdown by the same amount of summer export production at a higher Revelle factor at the end of the 21st century. This strongly increases the importance of the biological carbon pump in the entire Southern Ocean. In the temperate zone, between 30 and 44° S all models show a predominance of the warming signal and a nutrient-driven reduction of export production. As a consequence, the share of the regions south of 44° S to the total uptake of the Southern Ocean south of 30° S is projected to increase at the end of the 21st century from 47 to 66% with a commensurable decrease to the north. Despite this major reorganization of the meridional distribution of the major regions of uptake, the total uptake increases largely in line with the rising atmospheric CO2. Simulations with the MITgcm-REcoM2 model show that this is mostly driven by the strong increase of atmospheric CO2, with the climate-driven changes of natural CO2 exchange offsetting that trend only to a limited degree (~10%) and with negligible impact of climate effects on anthropogenic CO2 uptake when integrated over a full annual cycle south of 30° S.
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Figure 1: Timeseries of CO2 flux (FCO2, positive = into the ocean, PgC/yr) in three subregions of the Southern Oceanhdl:10013/epic.45943.d010hdl:10013/epic.45943.d001
Figure 2: Delta Export production (a) and delta FCO2 (b), both in units of PgC/yr, calculated as the average for period 2081-2100 minus the average for 2012-2031hdl:10013/epic.45943.d011hdl:10013/epic.45943.d002
Figure 3: Timeseries of summer (DJF) FCO2 between 30 and 44° S in eight models, normalised to the start valuehdl:10013/epic.45943.d012hdl:10013/epic.45943.d003
Figure 4: Delta Export production (a, c) and delta FCO2 (b, d), both in PgC/yr, calculated as average in 2081-2100 minus average for period 2012-2031 for all individual models in the regions (a, b) 44-58 S and (c, d) <58 Shdl:10013/epic.45943.d013hdl:10013/epic.45943.d004
Figure 5: Summer (DJF) change between 2012-2031 and 2081-2100 for all models and regions: (a) export production, note that units are PgC/month, (b) MLD [m], (c) nutrient limitation factor for diatoms (dimensionless)hdl:10013/epic.45943.d014hdl:10013/epic.45943.d005
Figure 6: Changes in minimum (summer) mixed layer depth (MLD) between 2012-2031 and 2081-2100 for seven modelshdl:10013/epic.45943.d015hdl:10013/epic.45943.d006
Figure 7: Correlation between monthly time-series at start of simulation (2012-2031) of export production and FCO2hdl:10013/epic.45943.d016hdl:10013/epic.45943.d055
Figure 8: Mean change of (A) total FCO2 , (B) FCO2 due to climate effects on natural and anthropogenic carbon and (C) FCO2 due to increase of atmospheric CO2 between the periods 2012-2031 and 2081-2100 in REcoM2 in different seasonshdl:10013/epic.45943.d017hdl:10013/epic.45943.d008
Figure 9: Box model results. (A) Experiment 1: delta FCO2 due to perturbation of one parameter at a time from the state 2012-2031 to the state 2081-2100hdl:10013/epic.45943.d018hdl:10013/epic.45943.d009